In the field of plating metal substrates, for example, particularly a process for plating with a precious metal such as gold, selectively controlling the coverage area and thickness of an adherent deposit of such plating metal is a significant cost saving step, often representing the difference between a commercially feasible process and a mere laboratory phenomenon.
The known processes fall into two generally accepted categories: wet plating and dry plating. Electro-plating is a wet plating process. The process is most commonly carried out by forming an electrical circuit between the metallic workpiece and a spaced electrode, all while located in a liquid bath containing the material to be plated out. Note U.S. Pat. No. 4,427,498 to Wagner and U.S. Pat. No. 4,534,843 to Johnson, et al.
These conventional electroplating processes can be quite costly and complicated, and can be hazardous both to personal safety and the environment. Waste treatment and disposal are also significant practical problems. Further complications ensue when conventional electroplating is used for selective depositing of the metal onto predetermined areas of the workpiece or to remote areas which may not be electrically connected for conventional plating. For instance, recycling of expensive unused gold can lead to problems.
There are other conventional techniques for depositing a noble metal such as gold on a substrate. Evaporation of gold contacts is in principle a dry plating or deposition alternative to wet electroplating. It is not considered practical, however, because conventional methods require a high vacuum, non-selective deposition of unnecessary gold, which requires extensive recycling, are not easy to turn on and off for spot evaporation without heavily coating a shutter mechanism, and also produces a shadow effect and a poorly defined image. Furthermore, conventional boat evaporation of certain alloy compositions which are required for wear resistance may be difficult. The shortcomings of this technology illustrate the need for a method of metal deposition, in only specifically defined areas, with well-controlled thickness and uniformity, as well as the ability to maintain stoichiometry of the plating alloy.
U.S. Pat. No. 4,987,006, assigned to the assignee hereof, represented a significant breakthrough in the dry plating or cold welding of metals, such as gold and precious metals, onto a substrate. A significant aspect of the aforementioned patent was the recognition of the importance of providing a polymeric film, essentially transparent to the wavelength of the pulsed laser used in the operation, as support for both the metal to be transferred to the substrate and the absorptive polymer. While the process thereof resulted in a smooth and continuous deposited layer, it was discovered that the adhesion of the transferred material, such as gold, was not as effective as desired. Through a combination of a higher, but precise laser energy density window, and a unique multi-layered composite metal foil to be laser bonded, it was discovered that superior adhesion was achieved by the process of this invention. The improved process hereof, and the results achieved thereby, will become more apparent in the description to follow.